The Renin Angiotensin System (RAS) is a key regulator of cardiovascular and renal function. Although many studies have focused on the impact of extracellular angiotensin II and its receptors Angiotensin II type 1 (AT1R) and Angiotensin II type 2 (AT2R) on the cardiovascular system, others have reported that angiotensin II is also present in the intracellular compartment and can be released upon cell stretch to mediate cellular growth and/or apoptosis1-3. While many of the autocrine effects of this endogenous angiotensin store are believed to be mediated by plasma membrane angiotensin receptors, an intracellular RAS acting on nuclear angiotensin receptors has also been proposee4, 5.
The RAS influences cardiovascular function via nitric oxide regulation6-8. AT1R blockade increases nitric oxide (NO) and this increase is abolished by concomitant AT2R blockade, suggesting that the AT2R is important in NO production9. AT2R likely increases NO production via direct stimulation of Nitric Oxide Synthase (NOS)10 or indirectly through bradykinin-dependent mechanisms6, 7. Very recently, the intracrine activation of AT2R has been reported to increase the production of NO in isolated cortical kidney nuclei11.
Possible sources of NO coupled to angiotensin signalling include the three canonical NOS isoforms; neuronal (nNOS), inducible (iNOS), and endothelial NOS (eNOS)1. Additionally, there have been reports of a NOS isoform in mitochondria (mtNOS)13, 14. Though the unique identity of mtNOS is still controversial15, this mitochondria-specific isoform has been localized to the inner mitochondrial membrane, where it may regulate mitochondrial respiration13, 16, 17. While several studies have provided evidence that angiotensin receptors can couple to the canonical NOS isoforms7, 18, nothing is known about whether intracellular angiotensin II influences mitochondrial NO production or if it has any other effects on mitochondrial function.
Subcellular localization of a functional Mitochondrial Angiotensin System (MAS) coupled to angiotensin receptors was recently documented (U.S. Provisional Patent Application No. 61/231,529, the contents of which is incorporated herein by reference), which opens a new area of investigation into the regulation of mitochondrial function by angiotensin II-mediated intracrine signalling. The presence and function of a MAS was investigated via high resolution transmission immunoelectron microscopy, confocal imaging in live cells tracking the expression of angiotensin receptors and real time measurement of mitochondrial NO production and respiration in response to activation or inhibition of the receptor(s) in isolated mitochondria. Functional angiotensin II type 2 receptors are present on the mitochondrial inner membrane and are colocalized with endogenous angiotensin II. Activation of the mitochondrial angiotensin system (MAS) is coupled to mitochondrial nitric oxide production and can modulate respiration. The localization of AT2R in the mitochondrial inner membrane suggests its importance to nitric oxide production, which is believed to originate in the inner membrane through mtNOS19-21. Based on these recent findings of a functional mitochondrial angiotensin system and the beneficial effects of AT1R blockers on mitochondrial number and function, such a beneficial effect might be mediated via unopposed mtAT2R.
Mitochondria play a key role in the regulation of energy metabolism, reactive oxygen species (ROS) production and apoptosis, and so mitochondria provide an attractive drug target. Although systemic administration of angiotensin receptor blockers (ARBs) has been documented to improve mitochondrial functions, such administration is limited, often in older populations, by its impairment of renal function and its effects on blood pressure. Moreover, the need to use a high concentration of a drug to achieve an effective local concentration at the disease site often results in accompanying nonspecific toxic side effects.
Therefore, there remains a need to develop methods for selectively targeting the mitochondrial angiotensin receptor, mtAT2R, for therapeutic intervention for the treatment of cardiovascular, renal, neurological, and musculoskeletal disorders associated with aging and mitochondrial dysfunction.